Zeolite is a kind of silicate having pores in the crystal structure and is present as a number of kinds (types) different in crystal structure (pore structure), such as LTA, MFI, MOR, AFI, FER, FAU, DDR and the like. These zeolites have own adsorptivities, catalyst performances, ion exchangeabilities, etc. based on respective crystal structures and chemical compositions and are in use in various application fields such as adsorbent, catalyst (catalyst support), ion exchanger and the like.
In recent years, zeolite has been used for gas separation due to inherent pores. For example, DDR (Deca-Dodecasil 3R) type zeolite [see, for example, W. M. Meier, D. H. Olson, Ch. Baerlocher, Atlas of zeolite structure types, Elsevier (1996)] composed mainly of silica (SiO2) and containing, in the crystal structure, relatively small (in various zeolites) pores (pore diameter: 4.4×3.6 Å) of 8-membered oxygen ring is in use as a separating membrane for selective permeation and separation of gas [e.g. carbon dioxide (CO2)] from mixed gas.
In such gas separation, a gas separator comprising a porous substrate (e.g. ceramic) and a gas-separating layer formed on the surface of the porous substrate are widely used for its sufficient amount of gas permeation and for its improved mechanical strength.
The above gas separator can be produced, for example, by a method which comprises adhering a zeolite powder as seed crystals on the surface of a porous substrate composed of a ceramic or the like, dipping the zeolite-adhered substrate in a solution of raw materials for zeolite synthesis, containing a structure-directing agent, water, a silicon (Si) source, etc., conducting, in this state, a heat treatment (a hydrothermal synthesis of zeolite), and further conducting a heat treatment in an oxygen-containing atmosphere (see, for example, JP-2003-159518).
In the above method, however, it was found that defects such as cracks and the like were generated in the gas-separating layer formed on the surface of the porous substrate during the heat treatment (conducted for burning-out of the structure-directing agent remaining in the synthesized zeolite). It was also found that even if no defects were regenerated during the production of the gas separator, defects such as cracks and the like were generated during use of the gas separator for gas separation. The presence of such defects is undesirable because it reduces the gas-separating ability of gas separator significantly.